A turbomachine turbine comprises several stages each including a guide vane element formed in an annular array of fixed blades supported by a casing of the turbine and a rotor mounted so as to rotate downstream of the guide vane element in a cylindrical or frustoconical envelope formed by ring sectors attached circumferentially end-to-end to the turbine casing.
The ring sectors comprise at their upstream ends circumferential rims engaged with a slight axial clearance in a radially inner annular groove of an annular casing rail and held radially in this groove by a C-section annular locking member that is engaged axially from upstream on the casing rail and on the circumferential rims of the ring sectors. These sectors are held axially by abutment of their circumferential rims on upstream and downstream faces respectively of the groove of the rail.
The rims of the ring sectors are “decambered” relative to the casing rail and the locking member, that is to say that the rims of the ring sectors have a radius of curvature greater than that of the casing rail and of the locking member, which makes it possible to mount the rims of the ring sectors with a certain radial prestress between the bottom of the groove of the rail and the locking member and thereby to limit the axial movements of the rims of the ring sectors in the groove.
In operation, the differential thermal expansions of the ring sectors and of the casing cause an increase of this radial prestress that is applied in isolated zones of contact between the rims of the ring sectors and the casing rail. But this prestress disappears progressively over time by wear of the rims of the ring sectors and of the casing in these zones of contact. When this radial prestress is zero, the rims of the ring sectors may move axially in the groove of the casing and wear by friction the upstream and downstream faces of the casing groove.
When this wear exceeds a certain value, the rims of the ring sectors may, by moving downstream in the groove, disengage from the locking member, which has the result of tilting the ring sectors toward the axis of the turbine and risking contact between the ring sectors and the turbine rotor, likely to cause destruction of the ring sectors and the rotor.